In the related art, in a case where a brushless motor is driven by a 120° rectangular wave, there is a possibility that a control circuit which drives a switching element that is used for a positive electrode and that drives the brushless motor may malfunction when a negative voltage is generated at an input terminal of the brushless motor. This possibility is described with reference to FIG. 4 to FIG. 6.
FIG. 4 is a block diagram showing a control system of a motor drive apparatus. FIG. 5 is a view showing a commutation pattern in a 120° rectangular wave drive. FIG. 6 is a view showing a situation that occurs when a negative voltage is generated at an input terminal of a motor.
A motor 18 shown in FIG. 4 is a three-phase DC brushless motor. The motor 18 is an inner rotor type and includes a rotor including a buried permanent magnet including a pair of a N-pole and a S-pole. The motor 18 includes an armature coil (U-phase coil, V-phase coil, W-phase coil). One end of the U-phase coil is connected to a terminal 24, and the other end is connected to a terminal 25. One end of the V-phase coil is connected to the terminal 25, and the other end is connected to a terminal 26. One end of the W-phase coil is connected to the terminal 26, and the other end is connected to the terminal 24. In this way, the armature coil is connected by a delta connection in the motor 18.
A motor drive apparatus 37a that controls the motor 18 includes an inverter circuit 38 that controls power distribution with respect to the armature coil. The inverter circuit 38 is connected to the terminal 24, the terminal 25, and the terminal 26. The inverter circuit 38 includes a series circuit 38U in which a switching element 38a on the positive electrode side that is connected to a positive electrode of an external electric source 40 is connected in series to a switching element 38d on the negative electrode side that is connected to a negative electrode of the external electric source 40. The inverter circuit 38 includes a series circuit 38V in which a switching element 38b on the positive electrode side that is connected to a positive electrode of an external electric source 40 is connected in series to a switching element 38e on the negative electrode side that is connected to a negative electrode of the external electric source 40. The inverter circuit 38 includes a series circuit 38W in which a switching element 38c on the positive electrode side that is connected to a positive electrode of an external electric source 40 is connected in series to a switching element 38f on the negative electrode side that is connected to a negative electrode of the external electric source 40. In the inverter circuit 38, the series circuit 38U corresponds to the U-phase and is connected to the terminal 24. In the inverter circuit 38, the series circuit 38V corresponds to the V-phase and is connected to the terminal 25, and the series circuit 38W corresponds to the W-phase and is connected to the terminal 26.
That is, the inverter circuit 38 includes a plurality of switching elements 38a to 38f that separately connects or cuts off current supply routes from the external electric source 40 to the terminals (the terminal 24, the terminal 25, and the terminal 26) of the armature coil. The plurality of switching elements 38a to 38f are formed of a semiconductor element such as an FET.
Further, the motor drive apparatus 37a includes a control circuit 45a that controls the inverter circuit 38. The control circuit 45a is a microcomputer that includes a CPU, a RAM, a ROM, and the like. The control circuit 45a includes a control part 50a, a FET drive electric source 51, and a driver circuit 52.
The driver circuit 52 outputs a drive signal that controls gate terminals of the switching elements 38a to 38f separately.
The FET drive electric source 51 is an electric source for supplying electric power from the external electric source 40 to a buffer that controls the switching elements 38a, 38b, 38c among buffers included in the driver circuit 52.
The control part 50a reads out a power distribution pattern that corresponds to a Hall stage number stored in a ROM which the control circuit 45a has on the basis of the combination (Hall stage number) of output signals of detection sensors 41 to 43 and outputs a PWM command signal to the driver circuit 52 in accordance with the power distribution pattern. The driver circuit 52 outputs a drive signal that controls gate terminals of the switching elements 38a to 38f separately on the basis of the PWM command signal that is input from the control part 50a. 
The detection sensors 41 to 43 which the motor drive apparatus 37a has are formed of, for example, a Hall IC and output, when a rotor is rotated, rotation positions (rotation phases) of the rotor as output signals that correspond to the U-phase, the V-phase, and the W-phase individually to the control part 50a. The control part 50a recognizes the Hall stage number on the basis of the output signal that is input from the detection sensors 41 to 43, reads out a power distribution pattern that corresponds to the Hall stage number, performs shifting by an amount (for example, electric angle of 30°) that corresponds to a predetermined rotation phase, and then outputs a PWM command signal to the driver circuit 52 in accordance with the power distribution pattern that is read out.
Thereby, each of the switching elements 38a to 38f is driven by the PWM control and is turned on or turned off intermittently in a period of time that corresponds to each power distribution pattern.
For example, as shown in FIG. 5, when the Hall stage number HS2 in which output signals of the detection sensors 41 to 43 are (ON (high level), OFF (low level), OFF) is switched to the Hall stage number HS3 in which the output signals of the detection sensors 41 to 43 are (ON, ON, OFF), after an electric angle of 30° from the switching, the power distribution pattern is switched from TP2a to TP3a. 
In a period of time that corresponds to the power distribution pattern TP2a, since the switching elements 38a, 38d are (ON, OFF), power is distributed to the terminal 24 to the U-phase coil, the terminal 24 is in a high level, and the U-phase coil is in a phase power distribution period. Since the switching elements 38b, 38e are (OFF, OFF), power is not distributed to the terminal 25 to the V-phase coil, and the V-phase coil is in a phase open period (floating level period). Since the switching elements 38c, 38f are switched alternately between ON and OFF, power is distributed to the terminal 26 to the W-phase coil, the terminal 26 is in a low level, and the W-phase coil is in a phase power distribution period.
In a period of time that corresponds to the power distribution pattern TP3a, since the switching elements 38a, 38d are (OFF, OFF), power is not distributed to the terminal 24 to the U-phase coil, and the U-phase coil is in a phase open period. Since the switching elements 38b, 38e are (ON, OFF), power is distributed to the terminal 25 to the V-phase coil, the terminal 25 is in a high level, and the V-phase coil is in a phase power distribution period. Since the switching elements 38c, 38f are switched alternately between ON and OFF, power is distributed to the terminal 26 to the W-phase coil, the terminal 26 is in a low level, and the W-phase coil is in a phase power distribution period.
An example is described about a possibility that the control circuit 45a which drives the switching element 38a (switching element that is used for a positive electrode and that drives the brushless motor) may malfunction when the power distribution pattern TP2a is switched to the power distribution pattern TP3a and when a negative voltage is generated at the terminal 24 (an input terminal of the brushless motor).
In FIG. 6, a terminal voltage of a motor line represents the voltage of the terminal 24. A flow-out current represents a current that flows from the FET drive electric source 51 to the terminal 24 via a buffer circuit that constitutes the driver circuit 52. A FET drive voltage represents an output voltage of the FET drive electric source 51.
After switching to the power distribution pattern TP3a, for a period of several hundred μsec or more, a reverse electromotive voltage (negative voltage) is generated. The several hundred μsec of the generation time is a period of about 100 to 800 μsec and represents a period of the power distribution pattern TP3a. 
A voltage difference (a negative voltage that is generated when the switching element is switched from ON to OFF when the power distribution pattern is changed from the power distribution pattern TP2a to the power distribution pattern TP3a) of a change from the terminal voltage of the motor line at the time of the power distribution pattern TP2a to the terminal voltage of the motor line at the time of the power distribution pattern TP3a becomes, for example, −0.5V or less, and thereby, an unexpected current flows out from the FET drive electric source 51. As a result, the supplied power exceeds an electric source supply capacity of the FET drive electric source 51, and the FET drive voltage is decreased (dropped). In a state where the FET drive voltage becomes insufficient, when the buffer of the driver circuit 52 drives the switching element 38a, a normal electric source voltage is not applied to the control circuit 45a that drives the switching element 38a, and therefore, there is a possibility that the switching element 38a may malfunction when being driven.
Such a period in which the FET drive voltage causes a voltage decrease is, as indicated by a region in FIG. 5 surrounded by a circle mark, a period in which a state where the switching elements 38c, 38f are (ON, OFF) of a period corresponding to the power distribution pattern TP6a is switched to a state where the switching elements 38c, 38f are (OFF, OFF) of a period corresponding to the power distribution pattern TPla, and the W-phase coil becomes in a phase open period. Further, as described above, the period in which the FET drive voltage causes a voltage decrease is a period in which a state where the switching elements 38a, 38d are (ON, OFF) of a period corresponding to the power distribution pattern TP2a is switched to a state where the switching elements 38a, 38d are (OFF, OFF) of a period corresponding to the power distribution pattern TP3a, and the U-phase coil becomes in a phase open period. Further, the period in which the FET drive voltage causes a voltage decrease is a period in which a state where the switching elements 38b, 38e are (ON, OFF) of a period corresponding to the power distribution pattern TP4a is switched to a state where the switching elements 38b, 38e are (OFF, OFF) of a period corresponding to the power distribution pattern TP5a, and the V-phase coil becomes in a phase open period.
In order to shorten such a period in which the FET drive voltage causes a voltage decrease, a motor drive apparatus that includes a function of reducing a period in which a negative voltage is generated at the input terminal of the brushless motor is required.
A motor drive apparatus that includes a regeneration means for allowing a current which is generated by a reverse electromotive force of a coil to flow to an electric source is described, for example, in Patent Document 1. However, the motor drive apparatus described in Patent Document 1 does not include a function of reducing a period in which a negative voltage is generated at an input terminal.